The present invention relates to an adaptive array antenna system in radio communication system for directivity control and waveform equalization.
An adaptive array antenna system controls directivity of an antenna system so that received waves which have high correlation with a desired signal are combined, and received waves which have low correlation with a desired signal are suppressed.
In an adaptive array antenna system, a directivity is controlled so that the square of an error between a receive signal and a reference signal is the minimum. If a directivity control of an adaptive array antenna system is ideally carried out, transmission quality is highly improved even under multi-path environment such as out of line-of-sight.
For comparison between a receive signal and a reference signal, synchronization of a receive signal must first be established. If synchronization is unstable, the operation of an adaptive array antenna itself becomes unstable. Therefore, the stable operation of synchronization is essential under severe environment with degraded transmission quality.
A prior adaptive array antenna system is shown in FIG. 34. This is for instance shown in R. A. Monzingo and T. W. Miller, Introduction to Adaptive Arrays, John Wiley & Sons, Inc. 1980.
An adaptive array antenna system comprises N number of antenna elements A511 through A51N, N number of complex weight means A521 through A52N for giving a weight to an output of each antenna element, a weight control A53 for control a weight of said complex weight means, a reference signal generator A54, and a combiner A55 for combining weighted signals.
A value of weight (W.sub.opt) for forming directivity so that the square of error between a desired signal and a receive signal is the minimum, is expressed in the equation (1), where signals received in N number of antennas are x1 through xN, weights in weight means A521 through A52N are w1 through wN, and d is a desired signal. EQU w.sub.opt =R.sub.xx.sup.-1 r.sub.xd (1)
where EQU R.sub.xx =E(x*x.sub.T) (2)
##EQU1##
In equations (2) and (3), R.sub.x is correlation matrix between antenna elements, E(P) is expected value of (P). The symbols x* and d* are conjugate of x and d, respectively. x.sup.T is transposed matrix of matrix x in the equation (4), and R.sub.xx.sup.-1 is inverse matrix of R.sub.xx. The equation (2) shows that the correlation matrix R.sub.xx between antenna elements is a product of a conjugate of a matrix x and a transposed matrix x.sup.T of a matrix x. In the equation (3), the value r.sub.xd is a matix of average of a product of a receive signal x1 through xN received by each antenna elements, and a conjugate of a desired signal component d.
In an adaptive array antenna system, a directivity is controlled so that an error between an output signal and a desired signal is the minimum. Therefore, the error is not the minimum until the directivity converges, and in particular, the error is large during the initial stage of the directivity control. when the error in the initial stage is large, carrier synchronization and timing synchronization are unstable, so that a frequency error and a timing error from a desired signal can not be detected. Thus, the value r.sub.xd might have large error, and an adaptive array antenna system does not operate correctly.
FIG. 35 shows a block diagram of a prior adaptive array antenna system having N number of antenna elements, and forming a directivity beam before synchronization is established. This is described in "Experiment for Interference Suppression in a BSCMA Adaptive Array Antenna", by Tanaka, Miura, and Karasawa, Technical Journal of Institute of Electronics, Information and Communication in Japan, Vol. 95, No. 535, pages 49-54, Feb. 26, 1996.
In the figure, the symbols A611 through A61N are a plurality of antenna elements, A621 through A62N are A/D converters each coupled with respective antenna element, A63 is an FFT (Fast Fourier Transform) multibeam forming means for forming a plurality of beams through FFT process by using outputs of the A/D converters A621 through A62N, A64 is a beam selection means for selecting a beam which is subject to weighting among the beams thus formed, and A65 is an adaptive beam control means for controlling a selected beam. The beam selection means A64 selects a beam which exceed a predetermined threshold, then, a directivity of an antenna is directed to a direction of a receive signal having high power. Thus, synchronization characteristcs are improved.
However, when signal quality is degraded because of long delay longer than one symbol length, and/or interference, no correlation is recognized between signal quality and receive level. In that environment, the prior art which forms a plurality of beams through FFT process, and selects a beam which exceeds a threshold, is not practical.
Further, the prior art which forms a plurality of beams through FFT process, and selects a beam which exceeds a threshold, needs much amount of calculation for measuring signal quality. Further, it has the disadvantage that an adaptive array antenna does not operate correctly because of out of synchronization in an indoor environment which generates many multi-paths.
Next, a prior art for establishing synchronization is described.
FIG. 36 shows a block diagram of a prior adaptive array antenna which uses a transversal filter. This is described in "Dual Diversity and Equalization in Digital Cellular Mobile Radio", Transaction on VEHICULAR TECHNOLOGY, VOL. 40, No. 2, May 1991.
In the figure, the numerals 14011 through 1401N are antenna elements, 1402 is a beam forming circuit, 14031 through 1403N are first weight means, 1404 is a first combiner, 1405 is a transversal filter, 14061 through 1406M are delay elements, 14070 through 1407M are second weight means, 1408 is a second combiner, 1412 is an automatic frequency control, 1413 is a timing regeneration circuit, 14141 through 1414N are A/D (analog to digital) converters.
FIG. 37 shows a detailed block diagram of first weight means 14031 through 1403N, and second weight means 14070 through 1407M. In the figure, 14091 through 14094 are multipliers for real values, 1410 is a subtractor for real values, and 1411 is an adder for real values. 1415 is a clock generator.
The timing regeneration circuit 1413 regenerates a clock signal which is the same as that of a receive signal. The A/D converters 14141 through 1414N carry out the A/D conversion of a receive signal by using the regenerated clock signal, and the converted signal is applied to the beam forming circuit 1402.
Assuming that an output signal of the beam forming circuit 1402 is y.sub.b (t), the weights cO through cM of the second weight means 10470 through 1047M are determined so that the following equation is satisfied. EQU C=R.sub.t.sup.-1 r.sub.txd (5)
where R.sub.t is matrix having (M+1) columns and (M+1) lines, having an element on i'th line and j'th column;
E[y.sub.b (t-(i-1)(T.sub.S /a)y.sub.b (t-(j-1)(T.sub.S /a)*] (6)
and r.sub.txd is a vector of (M+1) dimensions, having i'th element; EQU E[y.sub.b (t-(i-1)(T.sub.S /a)d(t)*]
where Ts is symbol length of a digital signal, and (a) is an integer larger than 2.
In the above prior art, a signal at each antenna elements is essential, and therefore, a receive signal at an antenna element is converted to digital form by using an A/D converter. However, if sampling rate in A/D conversion differs from receive signal rate, the algorithm of minumum mean square error can not be used at a beam forming network, since a beam forming circuit would be controlled by a data with no timing compensation.
Further, the prior art has the disadvantage that the operation is unstable, since waveform equalization is carried out in both a transversal filter and a beam forming circuit. Further, as the second weight means operates with complex values, the hardware structure is complicated.
Accordingly, it should be appreciated that the transmission quality would considerably be degraded and timing synchronization would be degraded, because of long delay longer than one symbol period in a digital radio circuit.
When timing synchronization is degraded in a prior art, a minimum mean square error algorithm can not be used, and an adaptive array antenna does not operate correctly.